The present invention relates to a device and a method for regulating an alternator according to the generic part of the main claim.
At present, three-phase alternators are predominantly used for supplying power in a motor vehicle. These alternators produce alternating voltage which is rectified in a rectifier arrangement downstream of the alternator and used for supplying the consumers and for charging the battery. Since the output voltage of the alternator depends to a considerable extent on the rate of rotation of the alternator, a voltage regulator is used for achieving a desired constant output voltage. The voltage regulator influences the exciter current of the alternator in such a way that the alternator output voltage maintains a constant value, e.g. 14 volts, in spite of substantial fluctuations in alternator speed.
Since the number of electrical consumers in the motor vehicle continues to grow steadily, the demands made on alternators continue to increase. Problems occur in the power supply particularly in winter when windshield heaters are switched on in addition to the other consumers or when the vehicle is operated over long periods of time exclusively in city traffic, that is, at lower speeds. For this reason, the attempt is made to increase the current delivery of three-phase alternators in such a way that the output of the alternator is increased while the voltage remains constant.
One possibility for increasing the alternator output is known from DE-OS 31 42 878. In this suggestion, the exciter winding is supplied with a voltage greater than the battery voltage. This increases the intensity of the exciter current so that a substantially greater magnetic field is generated which leads to an increase in the alternator output, all other alternator dimensions remaining the same. The maximum exciter current or maximum magnetic flux is determined by the thermal limiting values of the rectifier diodes or of the stator in the alternator. If the exciter winding is operated at a voltage which is greater than the vehicle supply voltage, as is described in DE-OS 31 42 878, there is a risk that the allowable thermal limiting values will be exceeded under unfavorable operating conditions.
To solve this problem it is suggested in DE-P 38 43 163 to increase the exciter current above the nominal exciter current under certain conditions in a three-phase alternator of a vehicle for optimal vehicle power supply. If the thermal limiting value is reached, this is promptly detected by temperature measurement means in the regulator due to the arrangement of the regulator on a structural component part of the alternator which becomes particularly hot. After this is detected, the exciter current is reduced until the thermal limiting value is not exceeded. However, DE-P 38 43 163 does not indicate how to effect the increase in the exciter current above the nominal regulator current.
The EP application 0 462 503, which is not a prior publication, is a further development of the application mentioned above. Its disclosure is repeated in the following description and essential points are elucidated.